1. Field of the Invention
The present invention relates to a heating device, an image forming apparatus, and an electric-power control method. More particularly, the invention relates to a heating device which is suitably applied to an electrophotographic apparatus, an electrostatic recording apparatus or the like which includes a heating device of a belt heating type as an image heating device, to an image forming apparatus, and to an electric-power control method.
2. Description of the Related Art
Conventionally, an image heating device (fixing device) is mounted in an image forming apparatus. A description will now be provided of a conventional image heating device (fixing device) which is mounted in an image forming apparatus, such as a copier, a printer or the like, and heats and fixes a toner image formed on a recording material.
In an image forming apparatus, such as a copier, a printer or the like, a heat-roller-type device has been widely used as a fixing device for heating and fixing an unfixed image (toner image) representing object image information formed and carried on a recording material (a transfer sheet, an Electrofax sheet, electrostatic recording paper, an OHP (overhead projector) sheet, printing paper, format paper or the like) according to image transfer or directly using means for an appropriate image forming process, such as an electrophotographic recording process, an electrostatic recording process, a magnetic recording process or the like, as a permanent fixed image on the surface of the recording material. Recently, heat-roller-type devices have been practically used from the viewpoint of quick starting and energy saving. Electromagnetic-induction-heating-type devices have also been proposed. Various types of fixing devices used in respective image forming apparatuses will now be described.
(a) Heat-roller-type Fixing Device
A heat-roller-type fixing device is basically configured by a pair of pressure-contact rollers, i.e., a fixing roller (heating roller) and a pressing roller. The pair of rollers are rotated, a recording material having an unfixed toner image to be fixed formed and carried thereon is guided into a fixing nip portion where the rollers are in pressure contact with each other and is grasped and conveyed at the fixing nip portion, and the unfixed toner image is fixed on the surface of the recording material by the heat of the fixing roller and the pressure at the fixing nip portion.
In general, the fixing roller includes a hollow metal roller made of aluminum as a base material (core), and a tungsten halogen lamp incorporated therein as a heat source. The fixing roller is heated by the heat of the tungsten halogen lamp, and the outer circumferential surface of the fixing roller is maintained at a predetermined fixing temperature by controlling current supply to the tungsten halogen lamp.
Particularly, a fixing device of an image forming apparatus for forming a full-color image, for which a capability of mixing toner images of four layers at maximum by sufficiently heating and fusing the images is required, includes a core having a high heat capacity, and a rubber elastic layer for uniformly fusing the toner images, provided at the outer circumferential surface of the core. The toner images are heated via the rubber elastic layer. In some fixing devices, a heat source is also provided within the pressing roller, and the pressing roller is heated and the temperature of the pressing roller is controlled.
However, in the heat-roller-type fixing device, even if the power supply of the image forming apparatus is turned on and power supply to the tungsten halogen lamp, serving as the heat source of the fixing device, is simultaneously started, a considerable time (a waiting time) is required until the temperature of the fixing roller reaches a predetermined fixable temperature from a completely cooled state, because the heat capacity of the fixing roller is large, resulting in an inferior quick starting property. In order to execute an image forming operation at any time even in a standby state (a non-image-output state) of the image forming apparatus, it is necessary to maintain the fixing roller at a predetermined temperature-control state by supplying power to the tungsten halogen lamp, resulting in large power consumption.
Particularly, when using a fixing roller having a large heat capacity, such as the fixing device of the above-described full-color image forming apparatus, since there is a delay until the temperature of the surface of the fixing roller reaches the temperature set by temperature control, problems such as insufficient fixing, unevenness in the gloss of the fixed image, offset and the like, arise.
(b) Film-heating-type Fixing Device
Film-heating-type fixing devices have been proposed, for example, in Japanese Patent Application Laid-Open (Kokai) Nos. 63-313182 (1988), 2-157878 (1990), 4-44075 (1992), and 4-204980 (1992).
That is, a nip portion is formed by inserting a heat-resisting film (fixing film) between a ceramic heater, serving as a heating member, and a pressing roller, serving as a pressing member. By introducing a recording material having an unfixed toner image to be fixed formed and carried thereon between the film and the pressing roller at the nip portion and grasping and conveying the recording material together with the film, the heat of the ceramic heater is given to the recording material via the film at the nip portion, and the unfixed toner image is fixed on the surface of the recording material by heat and pressure by the pressing force at the nip portion.
The film-heating-type fixing device has the advantages that, for example, an on-demand-type device can be provided by using low-heat-capacity members as the ceramic heater and the film, a state in which the device is heated to a predetermined fixing temperature may be provided by supplying power to the ceramic heater, serving as the heat source, only during image formation by the image forming apparatus, a waiting time from the turning-on of the power supply of the image forming apparatus to a state in which image formation can be executed is short (a quick starting property), and power consumption in a standby state is very small (power saving). However, the device of this type has a problem from the view point of the quantity of heat as a fixing device for a full-color image forming apparatus or a high-speed image forming apparatus requiring a large quantity of heat.
(c) Electromagnetic-induction-heating-type fixing device
Japanese Utility Model Application Laid-Open (Kokai) No. 51-109739 (1976) has disclosed an induction heating fixing device for heating a fixing roller by Joule heat generated by inducing current by a magnetic flux. This device can directly heat the fixing roller by utilizing the generation of an induction current, so that a fixing process having higher efficiency than the heat-roller-type fixing device using a tungsten halogen lamp as a heat source is achieved.
However, since the energy of an AC magnetic flux generated by an exciting coil, serving as magnetic-field generation means, is used for raising the temperature of the entire fixing roller, radiation loss is large. As a result, the ratio of the fixing energy to the input energy is low, thereby causing low efficiency.
Accordingly, high-efficiency fixing devices have been devised, for example, by reducing the distance between the exciting coil and the fixing roller, serving as a heating member, or concentrating the distribution of AC magnetic fluxes of the exciting coil in the vicinity of the fixing nip portion, in order to obtain high-density energy used for fixing.
A description will now be provided of the schematic configuration of an electromagnetic-induction-heating-type fixing device whose efficiency is improved by concentrating the distribution of AC magnetic fluxes of the exciting coil in the vicinity of the fixing nip, with reference to FIG. 2 which will be used in a first embodiment of the present invention. In FIG. 2, a cylindrical fixing film 10 serves as an electromagnetic-induction-heating rotating member having an electromagnetic-induction heating layer (including a conductive layer, a magnetic layer and a resistive layer). The cylindrical fixing film 10 is loosely fitted to the outer surfaces of gutter-shaped film guide members 16, each having a substantially semicircular cross section. Magnetic-field generation means 15 is disposed within the film guide members 16, and includes an exciting coil 18 and an E-shaped magnetic core 17. An elastic pressing roller 30 is in pressure contact with the lower surfaces of the film guide members 16 via the fixing film 10 while forming a fixing nip portion N having a predetermined width with a predetermined pressing force. The magnetic core 17 of the magnetic-field generation means 15 is disposed at a position corresponding to the fixing nip portion N.
The pressing roller 30 is rotatably driven in a counterclockwise direction indicated by an arrow by driving means M. A rotational force operates on the fixing roller 10 due to a frictional force between the pressing roller 30 and the outer surface of the fixing film 10 caused by the rotatable driving of the pressing roller 30, so that the fixing film 10 is rotated along the film guide member 16 with a circumferential speed substantially corresponding to the rotational circumferential speed of the pressing roller 30 in a clockwise direction indicated by an arrow, while the inner surface of the fixing film 10 tightly contacts the lower surface of the film guide member 16 at the fixing nip portion N (a pressing-roller driving method). The film guide members 16 press against the fixing nip portion N, support the exciting coil 18 and the magnetic core 17 constituting the magnetic-field generation means 15, support the fixing film 10, and stabilize the conveyance of the fixing film 10 during its rotation. The film guide member 16 is an insulating member which does not hinder passage of a magnetic flux, and is made of a material which can endure a high load.
The exciting coil 18 generates an AC magnetic flux by an AC current supplied from an exciting circuit (not shown). The AC magnetic flux is distributed so as to concentrate on the fixing nip portion N by the E-shaped magnetic core 17 provided at a position corresponding to the fixing nip portion N, and generates an eddy current in the electromagnetic-induction heating layer of the fixing film 10. The eddy current generates a Joule heat in the electromagnetic-induction heating layer due to the specific resistance of the electromagnetic-induction heating layer. The electromagnetic-induction heat of the fixing film 10 is generated so as to concentrate on the fixing nip portion N where the AC magnetic field is concentrated, so that the fixing nip portion N is very efficiently heated. The temperature of the fixing nip portion N is maintained at a predetermined temperature by controlling current supply to the exciting coil 18 by a temperature control system including temperature detection means (not shown).
In a state in which the pressing roller 30 is rotatably driven, the cylindrical fixing film 10 is thereby rotated along the film guide members 16, and the temperature of the fixing nip portion N is raised to the predetermined temperature due to the electromagnetic-induction heating of the fixing film 10 as a result of current supply from the exciting circuit to the exciting coil 18 in the above-described manner, a recording material P having an unfixed toner image t formed thereon which has been conveyed from image forming means (not shown) is guided to the fixing nip portion P between the fixing film 10 and the pressing roller 30 in a state in which the image surface is placed upward, i.e., the image surface faces the surface of the fixing film 10, and is grasped and conveyed through the fixing nip portion N together with the fixing film 10 in a state in which the image surface tightly contacts the outer surface of the fixing film 10 at the fixing nip portion N. While the recording material P is grasped and conveyed through the fixing nip portion N together with the fixing film 10, the unfixed toner image t on the recording material P is fixed by being heated by the electromagnetic-induction heating of the fixing film 10. After passing through the fixing nip portion N, the recording material P is separated from the outer surface of the rotating fixing film 10, and is conveyed and discharged.
In the above-described conventional configuration, however, if electric power is supplied to the fixing unit in a state in which the induction heating coil (exciting coil) and the fixing film are cool, a large current flows in the circuit because the skin resistance due to the wire of the induction heating coil and the resistance of a metal constituting a sleeve, serving as a load, is small. Hence, a switching device allowing large current must be used, resulting in an increase in the production cost. Furthermore, as the temperatures of the induction heating coil and the sleeve rise, the current decreases due to an increase in the resistance caused by the temperature rise. As a result, it is impossible to stably supply the same electric power, and the rise time increases.
The present invention has been made in consideration of the above-described problems.
It is an object of the present invention to provide a heating device, an image forming apparatus and an electric-power control method which can maintain the maximum electric power to a constant value when heating a metal fixing film according to a magnetic-induction-heating fixing method, prevent reduction in output due to a temperature rise, realize higher-speed and more stable on-demand fixing, secure safety, and allow urgent stop of a heating fixing operation.
According to one aspect, the present invention which achieves the above-described object relates to a heating device for heating a material to be heated according to a magnetic-induction heating method, including magnetic-field generation means for generating a magnetic field, an electromagnetic-induction heating member for performing electromagnetic-induction heating by the magnetic field generated by the magnetic-field generation means, current detection means for detecting a current flowing in the magnetic-field generation means, control means for controlling electric power supplied to the magnetic-field generation means, based on a result of detection of the current detection means, and current limitation means for limiting a maximum current flowing in the magnetic-field generation means based on the result of detection of the current detection means.
If such a heating device is mounted in a copier or a printer for performing image formation by heating and fixing an unfixed image on a material to be heated as a permanent fixed image using a resonance-type power supply, since electric power is controlled using a signal from the current detection means, it is possible to maintain the maximum electric power to a constant value when heating electromagnetic-induction heating means (a metal fixing belt as a specific example) according to a magnetic-induction-heating fixing method, prevent reduction in output caused by a temperature rise, and realize higher-speed and more stable on-demand fixing.
It is preferable that the heating device further includes temperature detection means for detecting a temperature at a portion near a contact portion between the electromagnetic-induction heating member and the material to be heated, and that the control means controls supply of electric power to the magnetic-field generation means so that the temperature detected by the temperature detection means equals a specified temperature.
Accordingly, since supply of electric power to the magnetic-field generation means is stopped when the detected temperature of a contact portion between electromagnetic-induction heating means and pressing means exceeds the specified temperature, it is possible to secure safety of the fixing device from thermal runaway.
It is preferable that the heating device further includes urgent stop means for urgently stopping a heating operation by the electromagnetic-induction heating means based on an external input.
Accordingly, since a heating operation by the electromagnetic-induction heating means is urgently stopped based on an external input, it is possible to urgently stop a fixing operation in an emergency.
According to another aspect, the present invention which achieves the above-described object relates to an image forming apparatus including a heating device for heating a recording material by heat generated according to a magnetic-induction heating method. The heating device includes magnetic-field generation means for generating a magnetic field, an electromagnetic-induction heating means for heating the recording material by performing electromagnetic-induction heating by the magnetic field generated by the magnetic-field generation means, and pressing means for grasping the recording material between the electromagnetic-induction heating means and the pressing means in a state of pressure contact with the electromagnetic-induction heating means. Electric power supplied to the magnetic-field generation means is controlled based on a result of detection of a current flowing in the magnetic-field generation means.
According to still another aspect, the present invention which achieves the above-described object relates to an electric-power control method applied to a heating device for heating a material to be heated according to a magnetic-induction heating method. The heating device includes magnetic-field generation means for generating a magnetic field, electromagnetic-induction heating means for heating the material to be heated by performing electromagnetic-induction heating by the magnetic field generated by the magnetic-field generation means, and pressing means for grasping the material to be heated between the electromagnetic-induction heating means and the pressing means in a state of pressure contact with the electromagnetic-induction heating means. The method includes the step of controlling electric power supplied to the magnetic-field generation means based on a result of detection of a current flowing in the magnetic-field generation means.
The foregoing and other objects, advantages and features of the present invention will become more apparent from the following description of the preferred embodiments taken in conjunction with the accompanying drawings.